1. Field of the Invention
The present invention relates to a vibration wave motor driven by a travelling vibration wave, and more particularly to a structure of a vibration member which propagates the vibration wave.
2. Description of the Prior Art
As shown in U.S. Pat. No. 4,019,073, a vibration wave motor transduces a vibration generated when a periodic voltage such as an A.C. voltage or a pulsating voltage is applied to electrostrictive elements, to a rotating motion or a linear motion. Since it needs no wiring, unlike a conventional electromagnetic motor, it is simple and compact in structure, produces a high torque at a low rotating speed and has a low moment of inertia.
In the vibration wave motor shown in the U.S. Pat. No. 4,019,073, a movable member such as a rotor which contacts a vibration member is frictionally driven in one direction by a standing vibration wave generated in the vibration member. In a forward movement of the vibration, the vibration member and the movable member frictionally contact each other, and in a backward movement, they separate from each other. Accordingly, the vibration member and the movable member must be structured to contact in small areas, that is, to present point or line contact to each other. As a result, friction drive efficiency is low.
Recently, in order to resolve the above problem, a vibration wave motor which friction-drives the movable member by a travelling vibration wave generated in the vibration member has been proposed.
FIG. 1 shows a major portion thereof. Numeral 1 denotes an electrostrictive element which may be PZT (titanic acid zirzonium lead) and numeral 2 denotes a vibration member which is supported on a stator (not shown) together with the electrostrictive elements 1. Movable member 3 is press-contacted to the vibration member 2 and forms a rotor. A plurality of electrostrictive elements 1 are bonded, and the electrostrictive elements in one group are arranged at a pitch which is shifted by one quarter of a wavelength .lambda. of a vibration wave, relative to the electrostrictive element of another group. In each group, the electrostrictive elements are arranged at a pitch of one half of the wavelength .lambda. and at opposite polarities between adjacent ones.
With the vibration wave motor of this structure, an A.C. voltage Vo sin .omega.T is applied to every other electrostrictive elements in one group and an A.C. voltage Vo cos .omega.t is applied to all electrostrictive elements in the other group. As a result, the electrostrictive elements are supplied with the A.C. voltages such that the polarities are opposite between adjacent ones and the phases are shifted by 90 degrees between the groups, and the electrostrictive elements vibrate. The vibration is propagated to the vibration member 2, which is bent in accordance with the pitch of the arrangement of the electrostrictive elements 1. The vibration member 2 projects at every other electrostrictive element position and recesses at every other electrostrictive element position. Since the electrostrictive elements in one group are arranged at one-quarter wavelength shifted positions relative to the electrostrictive elements in the other group, the bending vibration travels. While the A.C. voltages are applied, the vibrations are sequentially excited and propagated through the vibration member 2 as a travelling bending vibration wave.
The travel of the wave is shown in FIGS. 2A to 2D. Assuming that the travelling bending vibration wave travels in a direction X and O denotes a center plane of the vibration member in a stationary state, the vibration wave in a vibration state is shown by a chain line. In a neutral plane 6, bending stress is balanced. On a crossing line of the neutral plane 6 and a sectional plane 7 normal to the neutral plane 6, no stress is applied and a vertical vibration occurs. The sectional plane 7 makes a pendulum vibration laterally around the crossing line 5. In FIG. 2A, a point P on a crossing line of the sectional plane 7 and a surface of the vibration member 2 facing the movable member 1 is a right dead center of the lateral vibration and it makes only a vertical movement. In this pendulum vibration, a leftward (opposite to the direction of travel of the wave) stress is applied when the crossing line 5 is on a positive side of the wave (above the center plane O), and a rightward stress is applied when the crossing line 5 is on a negative side of the wave (below the center plane O). In FIG. 2A, a crossing line 5' and a sectional plane 7' show the former state in which a stress F' is applied to a point P', and a crossing line 5" and a sectional plane 7" show the latter status in which a stress F" is applied to a point P". As the wave travels and the crossing line 5 comes to the positive side of the wave as shown in FIG. 2B, the point P makes a leftward movement and an upward movement. In FIG. 2C, the point P is a top dead center of the vertical vibration and makes only the leftward movement. In FIG. 2D, it makes the leftward movement and a downward movement. As the wave further travels, it returns to the state of FIG. 2A through the rightward and downward movements and the rightward and upward movements. Through the series of movements, the point P makes a rotating elliptic motion, and a radius of rotation is a function of t/2 where t is a thickness of the vibration member 2. On the other hand, as shown in FIG. 2C, on a tangential line between the point P and the movable member 3, the movable member 3 is frictionally driven by the motion of the point P.
In this vibration wave motor, the velocity of the movable member 3 is determined by the velocity of the point P on the surface of the vibration member 2. In order to increase the velocity of the movable member 3, it is necessary to increase the rotating speed of the point P on the surface of the vibration member 2. The radius of the rotating vibration could be increased by increasing the thickness t of the vibration member 2, but as the thickness of the vibration member 2 increases, rigidity increases and vibration amplitude and the rotation angle of the rotating vibration are reduced. Therefore, there is a limit in the increase of the velocity of the point P on the surface of the vibration member 2.